The invention relates generally to self-contained power units particularly adapted to use in close association with a crop processing operation in which a crop residue may be used as a fuel and in particular to power units including an internal combustion engine simultaneously supplied with two or more fuels, the fuels differing substantially in physical properties and the fuel supply systems differing substantially in response to change of demand.
An unfavorable comparison of projected world energy needs with estimated availability, particularly of conventional non-renewable energy resources, has intensified interest in the development of alternative energy sources including fuels derived from wastes such as crop residues. Large amounts of energy are available in the form of crop, food processing and forest industry residues (biomass). However for successful utilization of this energy, particularly at the farm, processing plant or lumber mill, a simple low-cost method of recovering thermal and electrical energy from these residues is required. Downdraft gasification in a vertical flow packed-bed reactor is one such method and, because hot gases flow in the same direction as the fuel stock, has good potential to provide clean fuel for small engines but so far its adoption has been limited and embodied typically in batch-fired systems requiring considerable manual control and offering only limited running times. A practical system offering continuous automatically controlled operation and requiring a minimum of manual supervision and adaptable to providing a low BTU gas of a composition and condition suitable for conversion in an energy converter either as the sole fuel or as a fuel component in a more sophisticated multi-fuel system was still needed. Waste and residue conversion technologies, including pyrolysis systems utilizing vertical flow packed-bed reactors, are discussed in "Solid Wastes and Residues--Conversions by Advanced Thermal Processes", ed. Jones and Radding, ACS Sumposium Series, American Chemical Society 1978, through page 162.
Some studies of the utilization of producer gas from crop residues as a fuel in an internal combustion engine have been made with gas having a heat content of only 180 BTU's per cubic foot but it was projected that heating value could be increased to about 400 BTU's per cubic foot (at least from wood waste) if all condensible fractions were removed from the gas and if process air was reduced or eliminated from the gas. It has been suggested that producer gas, particularly that made from low quality and variable stock, is likely to be only marginally adequate for use as a sole fuel in an internal combustion engine even after a considerable refining and conditioning of the gas.
Known dual-fueled engines are typically supplied with piped gas under relatively high pressure, as well as diesel fuel, with a gas throttle valve responding directly to the engine governor's signals, If, however, gas for a dual-fueled engine comes directly from a local gas producer or gasifier, gas input response will be dependent upon the operating characteristics of the producer and its supporting equipment upstream of the engine. Slow response of this type of gas supply to a change in demand caused by a change in engine loading will result in large engine speed variations. If in a dual-fueled diesel engine the diesel injection quantity is fixed (say at a low pilot level) and the engine governor operates only the gas throttle valve, unacceptable variations of engine speed with engine load will generally result.
Another problem of known dual-fueled internal combustion engine control systems, where one of the fuels is a gas such as producer gas, is the hazard of explosion within the exhaust system if unburned producer gas is exhausted from the combustion chamber.